Date: Wed, 1 May 2024 17:57:20 +0000
From: Oliver Upton <oliver.upton@linux.dev>
To: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>
Cc: Marc Zyngier <maz@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
Subject: Re: [PATCH RFC] KVM: arm64: allow ID_MMFR4_EL1 to be writable
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Hi Russell,

On Wed, May 01, 2024 at 06:06:51PM +0100, Russell King (Oracle) wrote:
> Between 5.4 and 5.15, the guests view of HPDS, CnP, XNX and AC2
> changed their value on the same Neoverse N1 r3p1 hardware which makes
> migrating between these kernels on the host problematical.

It'd be helpful to expand a bit more on how these fields changed, better
yet if we can blame it back to a commit. I'm guessing the only direction
of migration you care about is old -> new then?

> We already permit changing HPDS in AA64MMFR1_EL1 and CnP in
> AA64MMFR2_EL1. We also allow LSM as we allow that in AA64MMFR2_EL1,
> so this patch includes support for that even though it isn't required.
> 
> Discussing with Marc Zygnier, AC2 should also be fine to be writable,

typo: Zyngier

> even though we don't inject an UNDEF if the guest accesses those
> registers.
> 
> The only questionable one is XNX, execute-never control distinction,
> which is also in AA64MMFR1_EL1 but we don't allow to be changed there.

It is quite odd for us to expose this field to non-nested VMs in the
first place, though I suppose we will apply an additional set of
restrictions for nested VMs when they come along.

-- 
Thanks,
Oliver